1. Field of the Disclosure
The present invention relates generally to power supplies, and in particular but not exclusively, relates to controllers for switch mode power supplies.
2. Background
A wide variety of household or industrial appliances require a regulated direct current (dc) source for their operation. Different types of switch mode power supplies are often utilized to convert low frequency (e.g., 50 Hz or 60 Hz) alternating current (ac) or high voltage dc input mains to a regulated dc output voltage at the output of the power supply. Switch mode power supplies are popular because of their small size due to high frequency operation, well regulated outputs, high efficiency, and the safety and protection features that are provided.
In general, a switch mode power supply includes a switching element accompanied with an energy transfer element, such as for example a high frequency transformer, which provides safety isolation. In many examples, the energy transfer element transforms the input voltage level to a lower output voltage level. The output of the transformer is then rectified and filtered to provide a regulated dc output to be provided to an electronic device. The controller for the switch mode power supply typically senses the output of the switch mode power supply in a closed loop to regulate the output.
Some common control methods used in the controllers for switch mode power supplies to regulate the output versus load and line variations are the pulse width modulation (PWM), pulse frequency modulation (PFM), ON-OFF control or pulse skipping.
One popular topology utilized for a switch mode power supply is a flyback converter. When the switch is closed in a flyback converter, energy is stored in the primary inductance of the energy transfer element because of the blocking direction of the secondary diode cannot transfer to the secondary winding and load. However, when switch opens, the stored energy is transferred to the load by the reversed direction of current. The output regulation of the power converter is through processing the feedback from output to generate an internal signal, which may be referred to as a control signal to regulate the output.
The feedback signal from the output can come through an opto-coupler from a sense circuit coupled to the dc output. This feedback is referenced to the secondary ground it is referred as the secondary control. In some switch mode power supplies, the output sense that generates the feedback or control signal may be extracted indirectly from a third winding that is magnetically coupled to the secondary winding on the same transformer core. In this example, the feedback signal may be referenced to the primary ground may therefore be referred to as the primary control. The third winding in some cases also provides the operating power for the power supply controller and is sometimes referred to as a bias or feedback winding. The feedback or control signal may then be used by the controller of the switch mode power supply to, for example, modulate the pulse width (i.e., PWM), change switching frequency (i.e., PFM) or disabling some switching cycles (pulse skipping) of a drive signal used to switch the power switch of the switch mode power supply to regulate the output.
Voltage mode control with fixed frequency and ON-time control, usually referred as pulse width modulation (PWM), is typically better suited for high load regulation while the pulse skipping control, also referred as burst mode, is typically utilized for regulation at low loads. Control methods such as fixed on-time, variable off-time or fixed off-time, variable on-time even though they result in switching period time change, would still fit in definition of PWM control.
Current mode control may utilize a fixed switching frequency. In current mode control schemes, the on-time of each pulse of the drive signal is terminated when the current flowing in the power switch reaches the current limit threshold of the pulse peak value. In this control method, power switch current ramps up linearly when the power switch is on until the power switch current reaches the current limit threshold. The power switch is then turned off and the current limit threshold is varied to regulate the output. The peak current control mode is also considered as PWM control. In some examples of switch mode power supply controller, in order to improve light load efficiency and no-load power consumption, the switching frequency and current limit level may be reduced in response to a load drop.